1. Field of the Invention
The invention relates to the field of implantable medical devices and to systems and methods of monitoring hemodynamic parameters to detect and determine the onset of various medical conditions, such as congestive heart failure and pulmonary edema.
2. Description of the Related Art
A variety of implantable medical devices are known to automatically monitor a patient's physiologic condition and to selectively provide therapy when indicated. Implantable pacemakers and/or cardioverter defibrillators (ICDs) are implantable medical devices configured to monitor a patient's cardiac activity and selectively provide therapy in response to detected cardiac arrhythmias. Cardiac arrhythmia are generally conditions in which the heart muscle contracts irregularly, e.g., faster or slower than normal. Implantable pacemakers and/or ICDs typically include a stimulation pulse generator that generates therapeutic stimulation for delivery to patient tissue. ICDs also typically include and are controlled by a microprocessor-based controller that regulates the delivery of such therapy.
Implantable pacemakers and/or ICDs are also typically configured to monitor the patient's cardiac activity, including a variety of hemodynamic parameters and cardiac signals, in order to detect abnormalities and cardiac arrhythmias. These devices also respond with an appropriate therapy when an abnormality is detected. For example, therapeutic electrical stimulation may be provided when an implantable pacemaker and/or ICD detects a cardiac arrhythmia.
The cardiac signals that are monitored to detect cardiac arrhythmias may also be useful in detecting other potential abnormalities that may be indicative of future heart problems. For example, a consistently low amplitude of the monitored cardiac signal may be indicative of a heart problem which can lead to future heart failure, if not detected early. Thus, the cardiac signals that are monitored can be stored and analyzed for early detection of other potential problems.
Implantable pacemakers, ICDs, and other devices can include sensors to detect any of a variety of hemodynamic parameters. In some cases these devices merely monitor the electrical activity of the heart; however, in many cases, these devices are also configured to detect other parameters, such as the fluid pressure within the heart itself.
Elevated pressure within the left atrium of the heart can indicate the onset of fluid accumulation in the lungs, which often indicates the onset of a condition known as congestive heart failure (CHF). Heart failure is a condition in which a patient's heart works less efficiently than it should due to the heart's failure to sufficiently supply the body with oxygen rich blood. Congestive heart failure is a condition where a person's heart fails to sufficiently supply oxygenated blood to the body accompanied by a build-up of fluid pressure in the pulmonary blood vessels that drain the lungs. Elevated left atrial pressure (LAP) can also lead to a condition known as pulmonary edema, which can cause shortness of breath, hypoxernia, acidosis, respiratory arrest, and death.
Mean left atrial pressure in healthy individuals is normally less than about twelve millimeters of mercury (12 mm Hg). Patients with CHF who have been medically treated and clinically “well compensated” may generally have mean left atrial pressures in the range from 15 to 20 mm Hg. Transudation of fluid into the pulmonary interstitial spaces can generally be expected to occur when the left atrial pressure is about 25 mm Hg. Transudation sometimes occurs at greater pressure levels, for example at more than about 30 mm Hg, in some patients with chronic CHF. Pulmonary edema has been found to be very reliable predicted by reference to left atrial pressures, and much less well correlated with conditions in any other chamber of the heart.
One or more transducers can be implanted at particular locations by an invasive or surgical procedure for the clinical purpose of measuring blood pressure within the body. These locations are well known to those skilled in the art, such as physicians and physiologists, and include the left ventricle, left atrium, pulmonary veins, pulmonary capillaries, the pulmonary arteries, the right ventricle, and the right atrium.
It has been appreciated for many years that, with the exclusion of certain well-known medical conditions, a key left-sided filling parameter determining the state of compensation of the left heart is the mean left atrial pressure (LAP). LAP is closely predicted by the pulmonary venous pressure (PVP), the pulmonary capillary wedge pressure (PCWP), or the pulmonary artery and left ventricular end diastolic pressures (PADP and LVEDP, respectively). Mean RA and end diastolic RV pressures can also be used to predict left sided decompensation. In addition, the RV pressure at the peak of the first derivative of pressure with respect to time during systolic contraction is sometimes used to estimate PADP.
These pressures are complex periodic time varying signals composed of the superposition of cardiac and respiratory component waves. During normal and certain pathologic conditions such as congestive heart failure, these mean or diastolic pressure indices typically vary from 0 to 40 mm Hg gauge pressure referenced to atmospheric pressure. With few exceptions, these pressure indices transiently decrease during inspiration and increase with exhalation coincident with changes in intrathoracic pressure caused by respiratory muscle contraction including the diaphragm and chest wall and lung elastic recoil. These pressure indices have exaggerated increases in response to sudden perturbations in intrathoracic pressure that can result from exaggerated breathing, coughing, sneezing or straining, etc.